Dear Olivier,
 
I have myself identified three polycerate sheep cranial fragments with primary and secondary horncores:
 
1) High Street, Leicester Romano-British late C1st/early C2nd AD.
2) Gaol Street, Hereford early post-medieval. In this specimen the main core is broken off and the secondary core still present.
3) Commercial Road, Hereford later medieval. Both horncores chopped off.
 
The High Street is unpublished and subject to an archive report I did for Leicestershire Archaeological Unit. The Hereford sites are due to be published in volume 5 of Hereford City Excavations (Baxter, I.L. Forthcoming. The faunal remains. In: Sherlock, H. Hereford City Excavations. Volume 5.)
 
Ian